1. Field of the Invention
The present invention relates to a magnetic plastic rotor disk manufacturing apparatus for production of flattened columnar magnetic plastic rotor disks suitably used for measuring instruments.
2. Description of the Prior Art
A magnetic plastic rotor disk b mounted on a rotor shaft a and having diametrically opposite N and S poles as shown in FIG. 8 has been used for measuring instruments for vehicles to detect or indicate the running speed of a vehicle and rotational frequency of the engine thereof. Magnetic flux density distribution on an outer circumferential surface of the magnetic plastic rotor disk b is required to vary sinusoidally in respect to the rotary angle thereof as shown in FIG. 9.
FIG. 10 shows a conventional magnetic plastic rotor disk manufacturing apparatus for manufacturing such a magnetic plastic rotor disk. In this magnetic plastic rotor disk manufacturing apparatus, a magnetic plastic material A containing a plastic as matrix is injected from an injection device B into a plurality of cavities E provided in a two-plate side gate type mold C through a sprue runner D. A yoke F provided with a coil, not shown, for creating a necessary magnetic field is disposed on a line extended from the plane parting surface C' of the mold C so as to receive the mold C therein to magnetize the magnetic powder contained in the magnetic plastic material A injected into the cavities E by applying the magnetic field thereto, so that the particles of the magnetic powder are magnetized in the same magnetic orientation.
Since this conventional magnetic plastic rotor disk manufacturing apparatus applies the magnetic field to the mold C so that the flux lines extend from one side to the other side of the same, there have been various problems cropped up such as; the cavities E facing the parting surface C' are not exposed uniformly to the magnetic field, the magnetic field is unable to act in the diametrical direction on all the cavities E due to magnetic interference between the cavities E, and thus the magnetic plastic rotor disk having the aforesaid desired surface magnetic flux density distribution characteristics cannot be obtained, whereby the magnetic plastic rotor disks are distorted or the zero-cross position of the magnetic plastic rotor disks is dislocated, and consequently the yield of the conventional magnetic plastic rotor disk manufacturing apparatus results unsatisfactory.
Furthermore, since the magnetic plastic rotor disk manufacturing apparatus needs a magnetic field creating device disposed in a limited space around the mold, the magnetic plastic rotor disk manufacturing apparatus is unable to employ a large magnetic field creating device having a large capacity of creating an intense magnetic field. Thus, the magnetic plastic rotor disk manufacturing apparatus is unable to manufacture magnetic plastic rotor disks having a high surface magnetic flux density. Still further, the disposition of the magnetic field creating device in a space around the mold entails difficulty in operating the magnetic plastic rotor disk manufacturing apparatus in combination with automated machines, such as an automatic insert feed machine, a molding extracting machine and a sprue runner removing machine.